Driving from Reno to Las Vegas Nevada Saturday, my wife went to pass a slower vehicle. As the front tires ran over the 'rumble strip' marking the centerline of the road, the vibration apparently affected the power steering system. A "Service Power Steering" message appeared on the dash and the power steering assist was suddenly gone causing very heavy manual steering. We pulled to the side of the road and called Onstar for a Volt Adviser. She set us up with an appointment with a Las Vegas Chevy dealer then we continued the 80 miles to Tonopah to get us out of the desert. All this time we did not shut the car down as it was still drive-able although hard to steer. We did not want to risk the vehicle not starting back up and being stuck waiting for a tow truck to come 200 miles from Las Vegas while we waited in the desert. We spent the night in Tonopah and the next morning, when the car started up, the power assist was back. My assumption is whatever monitors the power steering was confused by the rapid vibration from the rumble strip and interpreted it as a power steering system failure and shut the assist down. We decided not to go to the Vegas dealer but see what happens (car continues to work fine) and if necessary, return to our selling dealer if there is any further issues. Anyone else experience this?